Ship ramps such as a ro-ro (roll-on/roll-off) vessel or car ferry gate ramps are known to be driven by hydraulic systems. However, the use of hydraulic systems poses a potential risk of pollution or damage to goods in case there is a leakage of hydraulic oil used by the hydraulic system. Also, hydraulic systems may require to be continuously operated in order to keep the system pressurized, which consumes energy. Thus, electric systems are becoming more popular in ship ramp drives. Electric systems which drive the ship ramp by means of one or more electric motors can be simple to install and monitor during use and can also be maintenance-friendly. In addition, electric systems may provide considerable energy savings.
When the ship ramp is closing and near an upper end position (closed position), it may be desirable to reduce the electric motor maximum torque to ensure smooth closing of the ramp. With a reduced maximum torque, the seals on the ship ramp have a longer lifetime and there is less mechanical stress on the components. It is possible to reduce the maximum torque when the ramp is near the upper end position because then there is a reduced requirement for torque. The torque reduction in the proximity of the upper end position may be implemented by means of a proximity switch or sensor, by an operator command, and/or by controlling the motor supply system, e.g. an inverter, to limit the maximum torque of the motor.
When the ship ramp is opening and near a lower end position (open position), it may be desirable to put the ramp in tension or to a hold position (e.g., hanging on the harbor pier) without lifting the ramp up. This can be accomplished so that the maximum torque level of the electric motor is reduced to keep ramp winch wires tight and to take a part of the ramp load without lifting the ramp up. The torque reduction in the proximity of the lower end position may again be implemented by means of a proximity switch or sensor, by an operator command, and/or by controlling the motor supply system, e.g. an inverter, to limit the maximum torque of the motor.
However, in the aformentioned types of torque reduction, if for some reason, such as a defect in a proximity switch or a human error, the torque reduction is performed when the ramp is in a wrong position, e.g. midway between the end positions, the control of the ramp may be lost because of the reduced torque level and the ramp may, as a result, crash down uncontrollably. Especially in the case of heavy ramps such as ro-ro ship gate ramps, such an event may cause substantial damage to the equipment and also personal injuries.